System and method for positioning a product using a laser interferometer

ABSTRACT

A system for positioning a product, comprising a chuck for supporting the product, an intermediate stage supporting said chuck, and a stationary base supporting said intermediate stage. The chuck can move with respect to the intermediate stage in a first direction X, and the intermediate stage can move with respect to said stationary base in a second direction Y. The system furthermore comprises at least one laser interferometer for measuring the position of the chuck relative to the stationary base. The main part of the laser interferometer is attached to the intermediate stage, so that it can measure the distance between a reflector on the chuck and a reflector on the stationary base.

The invention is related to a system for positioning a product,comprising a chuck for supporting the product, an intermediate stagesupporting said chuck, and a stationary base supporting saidintermediate stage, whereby the chuck can move with respect to theintermediate stage in a first direction X, and the intermediate stagecan move with respect to said stationary base in a second direction Y,furthermore comprising at least one laser interferometer for measuringthe position of the chuck relative to the stationary base.

U.S. Pat. No. 5,757,160 discloses such system for accurately positioningand aligning a wafer as used in photolithography or microlithography insemiconductor manufacturing. The system comprises a plurality ofinterferometer laser gauges, each being the main part of a laserinterferometer, attached to the chuck (movable wafer stage) and at leastone elongated plane mirror reflector attached to the stationary base. Inorder to determine the position of the chuck, the distance between saidlaser gauge and the stationary reflector is measured by means of thelaser interferometer.

A laser interferometer, as referred to in this description, is generallyknown and comprises a main part, or laser gauge, which main part directsa laser beam towards one or more retro-reflectors. The retro-reflectorreflects the laser beam back to said main part, and the main partreceives the reflected laser beam. The length of the path of the laserbeam is determined by the laser interferometer, and therefore thedistance between said main part and said retro-reflector can bemeasured. Said main part of the laser interferometer may comprise knowncomponents such as a polarizing beam splitter, a quarter wave plate, anda cube corner reflector.

A polarizing beam splitter divides a laser beam, having two or morepolarization directions, into two polarized laser beams, each having acertain polarizing direction. Thereby a first laser beam is passing thesplitter in a straight path, and the other beam is directed in a certaindirection, in particular under an angle of 90° with respect to theoriginal beam.

A quarter wave plate rotates the direction of the polarization of apolarized laser beam over 45°, when such polarized laser beam is passingthrough such quarter wave plate.

A cube corner reflector is a retro-reflector provided with three planemirrors under an angle of 90° to each other, like in the corner of acube. A laser beam is reflected by a cube corner reflector in adirection parallel to the incident laser beam, however, in reversedirection and at a certain distance relative to said incident laserbeam.

In the known system for positioning a product, as is disclosed in U.S.Pat. No. 5,757,160, the distance between certain locations on the chuckand corresponding locations on the stationary base is measured, so thata quite exact determination of the position of the chuck is possible.However, in the known system the main part of each laser interferometeris attached to the chuck, and therefore such configuration requires arather large and heavy chuck, whereby furthermore electrical wires hasto be present between the chuck and the stationary base. As analternative, the main part of each laser interferometer can be attachedto the stationary base. However, in that case the chuck has to beprovided with a relatively large elongated plane mirror reflector. Thatreflector must be longer than the range of travel of the chuck, to makesure that the laser beam is caught by the reflector in each position ofthe chuck.

The object of the invention is to provide a system for positioning aproduct by means of laser interferometers, whereby the chuck is providedwith relatively small parts of the laser interferometers, and wherebylaser interferometers measure the distance between certain locations onthe chuck and corresponding locations on the stationary base.

In order to accomplish that objective, the main part of the laserinterferometer is attached to the intermediate stage, so that it canmeasure the distance between a reflector on the chuck and a reflector onthe stationary base. A laser interferometer for measuring the distancebetween two reflectors at both sides of the main part of theinterferometer is known. In case the main part of such laserinterferometer is attached to the intermediate stage, whereby themeasuring laser beam is parallel with said first direction X, that laserbeam will always hit the same location on the chuck, and therefore thatlocation of the chuck can be provided with a relatively smallretro-reflector.

Preferably, said reflector on the stationary base is an elongated planemirror reflector, having a length larger than the maximal displacementof the intermediate stage in said second direction Y, so that the laserbeam from the interferometer will hit that reflector in each position ofthe intermediate stage. In case such large plane mirror reflector isused, it is an advantage that such large reflector is attached to thestationary base, and not to any moving part of the system.

In one preferred embodiment, the main parts of two laser interferometersare attached to said intermediate stage, each for measuring the distancebetween a respective reflector on the chuck and the same elongated planemirror reflector on the stationary base. Thereby the measurement takesplace in the first direction X at one side of the chuck, so that theposition of the chuck in the first direction X is measured as well itsangular position relative to an axis in the third direction Z,perpendicular to the first direction X and the second direction Y. Ofcourse, apart from said two laser interferometers, also the main partsof other laser interferometers may be attached to the intermediatestage.

Preferable, the main parts of three laser interferometers are attachedto said intermediate stage, for measuring distances in the firstdirection X between one or more reflectors on the chuck and one or moreplane mirror reflectors in the stationary base. One large plane mirrorreflector can be attached to the stationary base and/or to the chuck.However, the chuck is preferably provided with three cube cornerreflectors, because such retro-reflectors are not sensitive for angularvariations of their positions. When the measurement takes place at threelocations that do not lie in a flat plane, the angular position of thechuck in the XY plane (i.e. around an axis in the third direction Z) aswell as in the XZ plane (i.e. around an axis in the second direction Y)can be determined.

Preferably, said reflector on the chuck is a cube corner reflector,whereby more cube corner reflectors may be attached to the chuck. Theadvantage of the cube corner reflector is its insensibility to smallangular variations, so that the laser beam is always reflected parallelto the incident laser beam, independent of angular variations of theposition of the chuck.

In one preferred embodiment, the main part of a laser interferometer isattached to said intermediate stage for measuring the distance in thethird direction Z between a reflector on the chuck and a reflector onthe stationary base, which direction Z is perpendicular to the firstdirection X and the second direction Y. Thereby the stationary base isprovided with a plane mirror reflector extending in a plane parallel tothe first direction X and the second direction Y, which plane mirrorreflector is elongated in the second direction Y. The chuck is providedwith a plane mirror reflector also extending in a plane parallel to thefirst direction X and the second direction Y, which reflector iselongated in the first direction X. The elongated reflector is attachedto the lower side of the chuck, which is not the side where the productto be positioned is located, and therefore there is no need to enlargethe chuck in order to attach that plane mirror reflector.

A system for measuring a distance by means of a laser interferometer,whereby the interferometer measures the distance between a plane mirrorreflector and a cube corner reflector, or between two cube cornerreflectors, can also be applied for other measurements, whereby thedistance between two objects has to be measured and whereby the angularposition one of the objects, or of both objects, may vary. Thereforethis measurement system can be seen as a separate invention, whichinvention can be applied independent of the application of otherinventions described in this description.

Such separate invention can be described as a system for measuring thedistance between two objects by means of a laser interferometer, wherebya part of the laser interferometer, comprising a laser beam splitter, acube corner reflector and two quarter wave plates, is located betweenthe two objects, whereby a retro-reflector is attached to each of theobjects, whereby one of the objects is provided with a plane mirrorreflector and whereby the other object is provided with a cube cornerreflector, or whereby both objects are provided with a cube cornermirror reflector.

The invention is furthermore related to a method for positioning aproduct by means of a system comprising a chuck for supporting theproduct, an intermediate stage supporting said chuck, and a stationarybase supporting said intermediate stage, whereby the chuck can move withrespect to the intermediate stage in a first direction X, and theintermediate stage can move with respect to said stationary base in asecond direction Y, furthermore comprising at least one laserinterferometer for measuring the position of the chuck relative to thestationary base, whereby the distance between a reflector on the chuckand a reflector on the stationary base is measured by means of a laserinterferometer, whereby the main part of that laser interferometer isattached to said intermediate stage.

In order to elucidate the invention, embodiments and portions of asystem for positioning a product by means of laser interferometers willbe described referring to the drawing, in which

FIG. 1 shows a laser interferometer for measuring a distance;

FIG. 2 is a top view of a first example of an interferometermeasurement;

FIG. 3 is a perspective view of a second example of an interferometermeasurement;

FIG. 4 is a top view of a third example of an interferometermeasurement; and

FIG. 5 shows an alternative laser interferometer for measuring adistance.

The figures are very schematic representations, whereby only relevantportions of the system for positioning a product by means of laserinterferometers are shown.

FIG. 1 shows a laser interferometer comprising a laser beam splitter 1,a cube corner reflector 2 and two quarter wave plates 3,4, also calledλ/4 plates. These components of the laser interferometer are attached tothe intermediate stage 5 of a system for positioning of a product. Theintermediate stage 5 can move in a second direction Y, as is indicatedwith arrow 6. The laser interferometer is located between a stationarybase 7 and a chuck 8, to which the product to be positioned can beattached. The chuck 8 can move in a first direction X with respect tothe intermediate stage 5, as is indicated with arrow 9. A plane mirrorreflector 10 is attached to the chuck 8 and an elongated plane mirrorreflector 11 is attached to the stationary base 7.

The distance between the stationary base 7 and the chuck 8 can bemeasured by means of the laser interferometer as follows. A polarizedlaser beam 12, comprising two polarization directions perpendicular toeach other is directed to the represented parts of the interferometer. Afirst part of the laser beam 12 follows a path including four times thedistance between the plane mirror reflectors 10,11, and the other partof the laser beam 12 follows a fixed path through the laserinterferometer. The distance between the two plane mirror reflectors10,11, and therewith the distance between the chuck 8 and the stationarybase 7, can be determined based on the difference in lengths of said twopaths followed by the two parts of the laser beam. The interferometercan measure that difference in length.

In the beam splitter 1, the first part of the incident laser beam 12 isturned off by an angle of 90° (arrow 13) to and through quarter waveplate 3 towards the plane mirror reflector 10 (arrow 14). The reflectedlaser beam (arrow 15) passes again the quarter wave plate 3, so that thedirection of the polarization of the laser beam (arrow 16) is rotatedover 90° (two times 45°) compared to the laser beam indicated with arrow13. Therefore, the laser beam (arrow 16) can pass the beam splitter 1 ina straight path towards quarter wave plate 4, where the direction ofpolarization rotates over 45°. Then the laser beam (arrow 17) isreflected against the plane mirror reflector 11 and returns at thequarter wave plate 4 (arrow 18), where the direction of polarizationrotates again over 45°, so that the beam (arrow 19) is rotated over 90°compared to the beam indicated with arrow 16. Therefore, the beam (arrow19) is turned off by an angle of 90° by the laser beam splitter 1 (arrow20) towards cube corner reflector 2.

In the cube corner reflector 2 the beam 20 is reflected (arrow 21) inorder to leave the reflector 2 (arrow 22) in reverse direction parallelto and at a distance from the incident beam 20. The reflected beam 22 isturned off by an angle of 90° (arrow 23) by the beam splitter 1 towardsquarter wave plate 4. When passing quarter wave plate 4 the direction ofthe polarization of the laser beam is rotated over 45° and afterreflection against plane mirror reflector 11 (arrows 24 and 25) thedirection of polarization is again rotated over 45° by the quarter waveplate 4, so that the laser beam (arrow 26) can pass the beam splitter 1in a straight path towards quarter wave plate 3. The direction ofpolarization of the beam is rotated over 45° and the beam (arrow 27) isdirected towards plane mirror reflector 10. After reflection by theplane mirror reflector 10 the laser beam (arrow 28) arrives again at thequarter wave plate 3, where the direction of polarization is againrotated over 45°, so that the total rotation compared to the beamindicated by arrow 26 is 90°. Therefore, the beam (arrow 29) is turnedoff by an angle of 90° by the beam splitter 1, whereby the beam iscombined again with said other part of the original laser beam 12 toform the laser beam 30 that leaves the relevant part of theinterferometer.

The other part of the laser beam 12 has a direction of polarization thatcan pass the laser beam splitter 1 in a straight path (arrow 31), andafter reflection by cube corner 2 (arrow 32) the other part of the laserbeam is directed towards the beam splitter 1 (arrow 33). The laser beam(arrow 33) passes again the beam splitter 1 in a straight path, so thatit forms a part of the laser beam 30 that leaves the relevant part ofthe laser interferometer.

The part of the laser interferometer that measures the difference inlength of the two paths that is followed by the two parts of the laserbeam is not represented in the figure. That part of the interferometeris located at a fixed position relative to the stationary base 7, sothat the distance between that part and the main part of theinterferometer that is attached to the intermediate stage 5 varies.However, such variation does not have influence on the difference inlength of said two paths, and therefore it has no influence on themeasurement results.

FIG. 2 is a top view of a positioning system showing a stationary base41 supporting an intermediate stage 42 that can move in the seconddirection Y (arrow 43) relative to the stationary base 41. Theintermediate stage 42 supports the chuck 44, which chuck 44 can supportthe product to be positioned. The chuck 44 can move relative to theintermediate stage 42 in the first direction X (arrow 45). FIG. 2 showsthe main parts of a laser interferometer for measuring the position ofthe chuck 44 in de first direction X with respect to the stationary base41. Therefore the distance 46 is measured by means of a laserinterferometer as is described above referring to FIG. 1.

The main part 47 of the interferometer comprises a beam splitter, a cubecorner reflector and two quarter wave plates, and is attached to theintermediate stage 42. A laser beam 48, being parallel to the seconddirection Y, is directed to said main part 47. As described above, afirst part of the laser beam 48 follows a path including four times thedistance between said main part 47 and a plane mirror reflector 49 onthe chuck and four times the distance between said main part 47 and anelongated plane mirror reflector 50 on the stationary base 41. Each ofthe four beams 51,52,53,54 represents a forward and a return path ofsaid first part of the laser beam 48 towards the plane mirror reflectors49 and 50 respectively.

The other part of the laser beam 48 follows a much shorter path having afixed length, as is described above referring to FIG. 1. The two partsof the laser beam 48 are combined again in the laser beam 55 that leavessaid main part 47 of the interferometer. The difference in lengths ofsaid two paths is determined by the interferometer, and thereby thedistance 46 is measured.

FIG. 3 shows an example whereby the position of the chuck relative tothe stationary base in the third direction Z is measured by means of alaser interferometer. The stationary base 61 supports the intermediatestage 62, which intermediate stage can move in the second direction Y(arrow 63). The intermediate stage 62 supports the chuck 64, which chuck64 can move in the first direction X (arrow 65) relative to theintermediate stage 62. The product to be positioned can be attached tothe chuck 64.

The stationary base 61 is provided with an elongated plane mirrorreflector 66 extending in the second direction Y, and the lower side ofthe chuck 64 is provided with an elongated plane mirror reflector 67extending the first direction X. Both plane mirror reflectors 66,67 areparallel to the first direction X and to the second direction Y. Inorder to measure the positioning of the chuck 64 relative to thestationary base 61, the main part 68 of the interferometer directs laserbeams towards the two plane mirror reflectors 66,67. Thereby theinterferometer functions in the same way as described above referring toFIG. 2. A laser beam 69 is directed to said main part 68 and a laserbeam 70 returns from it. A part of the laser beam 69,70 follows a pathincluding the distance between the two plane mirror reflectors 66,67,and the other part of the laser beam 69,70 follows a path having a fixedlength. The interferometer can determine the difference in length of thetwo paths and therewith the distance between the two plane mirrorreflectors 66,67 is measured.

FIG. 4 shows in top view a system for positioning a product by means ofa number of laser interferometers. Thereby the chuck 71 is substantiallysurrounded by the stationary base 72, and the position of the chuck 71relative to the stationary base 72 is measured by six laserinterferometers 73,74,75,76,77,78, so that the complete position of thechuck 71 (i.e. the location and the rotational position) can bedetermined.

The chuck 71 is supported by intermediate stage 79 and can move in thefirst direction X relative to intermediate stage 79, as is indicatedwith arrow 80. The intermediate stage 79 is supported by the stationarybase 72 and can move in the second direction Y relative to thestationary base 72, as is indicated with arrow 81. The product to bepositioned can be attached to the chuck 71 and the chuck 71 can bedisplaced, whereby the each position of the chuck can be determined bymeasurements by means of each of the six laser interferometers73,74,75,76,77,78.

The three laser interferometers 73,74,75 are attached to theintermediate stage 79 and can measure the distance between the elongatedplane mirror reflector 82 attached to the stationary base 72 and threerespective plane mirror reflectors 83,84,85 on the chuck 71. The threeinterferometers 73,74,75 are of the type described above and shown inFIG. 1. Each laser beam, comprising the beams towards and from each ofthe three interferometers, is indicated with reference numeral 86. Themeasurements of the three interferometers 73,74,75 are similar to themeasurement as described above referring to FIG. 2. The interferometer74 is located lower than the other two interferometers 73,75, so thatthe three plane mirror reflectors 83,84,85 are located at the corners ofa triangle, whereby reflector 84 is located lower than the other tworeflectors 83,85. Therefore the three interferometers 73,74,75 canmeasure the position of the chuck 71 in the first direction X and theangular positions around an axis in the second direction Y and around anaxis in the third direction Z.

The two laser interferometers 76,77 are also attached to theintermediate stage 79 and can measure the distance between elongatedplane mirror reflector 87 attached to the stationary base 72 and twoelongated plane mirror reflectors extending in the first direction X(not shown in the figure) at the lower side of the chuck 71. Bothinterferometers 76,77 are of the type described above and shown inFIG. 1. The measurements of the two interferometers 76,77 are similar tothe measurement as described above referring to FIG. 3. The laser beam88 comprises the laser beams towards and from interferometer 77, and thelaser beam 89 comprises the laser beams towards and from interferometer76, which beam is reflected by mirror 90 on its way to and from theinterferometer 76. The two interferometers 76,77 can measure theposition of the chuck 71 in the third direction Z and the angularposition around an axis in the first direction X.

Laser interferometer 78 is attached to the stationary base 72 and canmeasure its distance to the elongated plane mirror reflector 91 on thechuck 71. Interferometer 78 is of an conventional type. Arrow 92indicates the path of the measuring laser beam and the length of thatpath is measured. Beam 93 shows the laser beam towards and from theinterferometer 78.

In the above description the expression interferometer is used for themain part of that device, i.e. the part that directs the measuring laserbeam towards the retro-reflectors. The part of each interferometer thatmeasures the difference in length of the path of the measuring laserbeam and the fixed length of the path of the reference laser beam is notrepresented in the drawing.

FIG. 5 shows a laser interferometer similar to the interferometer shownin FIG. 1, however one of the two retro-reflectors is not a plane mirrorreflector, but a corner cube reflector 110. The advantage of the cubecorner reflector is its insensitivity for its angular position. In casethe chuck is provided with a cube corner reflector in stead of a planemirror reflector, the measurement is less sensitive for variations inthe angular position of the chuck, because the reflected laser beam ofthe cube corner reflector is always parallel with the incident laserbeam.

The laser interferometer shown in FIG. 5 comprises a laser beam splitter101, a cube corner reflector 102 and two quarter wave plates 103,104.These components of the laser interferometer are attached to theintermediate stage 105 of a system for positioning of a product. Theintermediate stage 105 can move in the second direction Y, as isindicated with arrow 106. The laser interferometer is located between astationary base 107 and a chuck 108, to which the product to bepositioned can be attached. The chuck 108 can move in the firstdirection X with respect to the intermediate stage 105, as is indicatedwith arrow 109. A corned cube reflector 110 is attached to the chuck 108and an elongated plane mirror reflector 111 is attached to thestationary base 107.

The distance between the stationary base 107 and the chuck 108 can bemeasured by means of the laser interferometer as follows. A polarizedlaser beam 112, comprising two polarization directions perpendicular toeach other is directed to the represented parts of the interferometer. Afirst part of the laser beam 112 follows a path including four times thedistance between the retro-reflectors 110,111, and the other part of thelaser beam 112 follows a fixed path through the laser interferometer.The distance between the two retro-reflectors 110,111, and position ofthe chuck 108 in the first direction X relative to the stationary base107, can be determined based on the difference in lengths of said twopaths followed by the two parts of the laser beam 112.

In the beam splitter 101, the first part of the incident laser beam 112is turned off by an angle of 90° (arrow 113) to and through quarter waveplate 103 towards the cube corner reflector 110 (arrow 114). The laserbeam passes the cube corner reflector 110 (arrow 115) and is reflected(arrow 116) towards the quarter wave plate 103. After passing thequarter wave plate 103, the direction of the polarization of the laserbeam (arrow 117) is rotated over 90° (two times 45°) compared to thelaser beam indicated with arrow 113. Therefore the laser beam (arrow117) can pass the beam splitter 101 in a straight path towards quarterwave plate 104, where the direction of polarization rotates over 45°.Then the laser beam (arrow 118) reflects against the plane mirrorreflector 111 and returns at the quarter wave plate 104 (arrow 119),where the direction of polarization rotates again over 45°, so that thebeam (arrow 120) is rotated over 90° compared to the beam indicated witharrow 117. Therefore the beam (arrow 120) is turned off by an angle of90° by the laser beam splitter 101 (arrow 121) in the direction of thecube corner reflector 102.

In the cube corner reflector 102 the beam 121 is reflected (arrow 122)in order to leave the reflector 102 (arrow 123) in reverse directionparallel to and at a distance from the incident beam 121. The reflectedbeam 123 is turned off by an angle of 90° (arrow 124) by the beamsplitter 101 in the direction of quarter wave plate 104. When passingquarter wave plate 104 the direction of the polarization of the laserbeam is rotated over 45°. The beam (arrow 125) then arrives at the planemirror reflector 111, and is reflected (arrow 126) to the quarter waveplate 104 where the direction of polarization again rotates over 45°, sothat the beam (arrow 127) passes beam splitter 101 in a straight path.After passing quarter wave plate 103 the beam (arrow 128) is reflectedin the cube corner reflector 110 (arrows 129 and 130) and arrives atquarter wave plate 103, where the direction of polarization is againrotated over 45°, so that the total rotation compared to the beamindicated by arrow 127 is 90°. Therefore, the beam (arrow 131) is turnedoff by an angle of 90° by the beam splitter 101, whereby the beam iscombined again with said other part of the original laser beam 112 toform the laser beam 132 that leaves the relevant part of theinterferometer.

The other part of the laser beam 112 has a direction of polarizationthat can pass the laser beam splitter 101 in a straight path (arrow133), and after reflection by cube corner 102 (arrow 134) the other partof the laser beam is directed again towards the beam splitter 101 (arrow135). The laser beam (arrow 135) passes again the beam splitter 101 in astraight path, so that it forms a part of the laser beam 132 that leavesthe relevant part of the laser interferometer.

As said above referring to FIG. 1, the part of the laser interferometerthat measures the difference in length of the two paths that is followedby the two parts of the laser beam is not represented in the figure.That part is located at a fixed position relative to the stationary base107, so that the distance between that part and the main part of theinterferometer that is attached to the intermediate stage 105 varies.However, such variation does not have influence on the difference inlength of said two paths, and therefore it has no influence on themeasurement results.

The embodiments as described above are merely examples of the system forpositioning a product by means of a laser interferometer; a great manyother embodiments are possible.

REFERENCE NUMBERS

-   1 laser beam splitter (=schuin staande ‘spiegel’)-   2 cube corner reflector-   3 quarter wave plate-   4 quarter wave plate-   5 intermediate stage-   6 arrow-   7 stationary base-   8 chuck-   9 arrow-   10 plane mirror reflector-   11 plane mirror reflector-   12 laser beam-   13-29 arrows-   30 laser beam-   31-33 arrows-   34-40 . . .-   41 stationary base-   42 intermediate stage-   43 arrow-   44 chuck-   45 arrow-   46 distance-   47 main part of interferometer-   48 laser beam-   49 plane mirror reflector-   50 plane mirror reflector-   51-55 laser beams (4×)-   56-60 . . .-   61 stationary base-   62 intermediate stage-   63 arrow-   64 chuck-   65 arrow-   66 plane mirror reflector-   67 plane mirror reflector-   68 main part of interferometer-   69 laser beam-   70 laser beam-   71 chuck-   72 stationary base-   73-78 laser interferometers-   79 intermediate stage-   80-81 arrows-   82-85 plane mirror reflectors-   86 laser beams (3×)-   87 plane mirror reflector-   88-89 laser beams-   90 mirror-   91 plane mirror reflector-   92 arrow-   93 laser beam-   94-100 . . .-   101 beam splitter-   102 cube corner reflector-   103-104 quarter wave plates-   105 intermediate stage-   106 arrow-   107 stationary base-   108 chuck-   109 arrow-   110 cube corner reflector-   111 plane mirror reflector-   112 laser beam-   113-131 arrows-   132 laser beam-   133-135 arrows

1. A system for positioning a product, comprising a chuck for supportingthe product, an intermediate stage supporting said chuck, and astationary base supporting said intermediate stage, whereby the chuckcan move with respect to the intermediate stage in a first direction X,and the intermediate stage can move with respect to said stationary basein a second direction Y, furthermore comprising at least a first and asecond laser interferometer for measuring the position of the chuckrelative to the stationary base, a first and a second main part of saidrespective first and second laser interferometers including opticalcomponents for receiving and directing a first and a second laserrespectively, the first and second main parts being attached to saidintermediate stage and being movable therewith for measuringrespectively the distance between a first elongated plane mirrorreflector on the chuck that is elongated in the first direction X and anelongated plane mirror reflector on the stationary base that iselongated in the second direction Y, and the distance between a secondelongated plane mirror reflector on the chuck that is elongated in thefirst direction X and the elongated plane mirror reflector on thestationary base.
 2. A system as claimed in claim 1, the elongated planemirror reflector on the stationary base having a length larger than themaximal displacement of the intermediate stage in said second directionY.
 3. A system as claimed in claim 1, further comprising a third laserinterferometer having a main part that is attached to said stationarybase, the main part including optical components for receiving anddirecting a third laser for measuring the distance between a thirdelongated reflector on the chuck that is elongated in the firstdirection X and the main part on the stationary base.
 4. A system asclaimed in claim 1, further comprising three laser interferometers eachhaving a main part, the respective main parts of the three laserinterferometers are attached to said intermediate stage and movabletherewith, for measuring distances in the first direction X between oneor more first reflectors on the chuck and one or more plane mirrorreflectors in the stationary base.
 5. A system as claimed in claim 1,the chuck further comprising a cube corner reflector.
 6. A system asclaimed in claim 1, wherein the first and second main parts are attachedto said intermediate stage for measuring respectively the distance inthe third direction Z between the first elongated plane mirror reflectoron the chuck and the elongated plane mirror reflector on the stationarybase, and the distance in the third direction Z between the secondelongated plane mirror reflector on the chuck and the elongated planemirror reflector on the stationary base, which third direction Z isperpendicular to the first direction X and the second direction Y.
 7. Amethod for positioning a product by means of a system comprising a chuckfor supporting the product, an intermediate stage supporting said chuck,and a stationary base supporting said intermediate stage, whereby thechuck can move with respect to the intermediate stage in a firstdirection X, and the intermediate stage can move with respect to saidstationary base in a second direction Y, the method comprising attachingat least a first and a second laser interferometer to the intermediatestage, the first and second laser interferometers respectively furthercomprising a first and a second main part including optical componentsfor receiving and directing a first and a second laser, the first andsecond main parts being movable with the intermediate stage, andmeasuring the position of the chuck relative to the stationary base bymeasuring a first distance between a first elongated reflector on thechuck and an elongated reflector on the stationary base using the firstlaser interferometer, and a second distance between a second elongatedreflector on the chuck and the elongated reflector on the stationarybase using the second laser interferometer.
 8. A method as claimed inclaim 7, wherein the first and second elongated reflectors on the chuckare elongated in the first direction X and the elongated reflector onthe stationary base is elongated in the second direction Y.
 9. A methodas claimed in claim 7, further comprising moving the chuck relative tothe stationary base and measuring the position of the chuck relative tothe stationary base during such movement.